A. Field of the Invention
This invention relates to the field of microelectronics and more particularly to means for electrically and structurally interconnecting microelectronic components.
B. Description of the Prior Art
A problem is created when microelectronic devices made from different materials are interconnected and then exposed to variations in temperature. The difference in thermal expansion between the materials causes stresses to be created between the different components which can cause failure of the interconnection and degradation of component characteristics. Until the present invention, this problem has prevented the direct interconnection of planar (two dimensional), high density arrays of microelectronic components which are exposed to temperature variations and are made from materials having different thermal expansion.
A technique called flip-chip bonding of silicon chips or dice is known. According to this technique, thin pads, or bumps, of metal are deposited on one side of a silicon chip and an appropriate substrate with interconnecting circuitry. The chip is then "flipped" over on top of the substrate and corresponding pads on the chip and substrate are bonded together. The resulting interconnections are not flexible and will rupture if exposed to thermal stresses. However, if both chip and substrate are made from materials having the same thermal expansion coefficient, significant thermal stresses are not created in the joint.
In some applications, it is necessary to join microelectronic components made from two different materials having different thermal expansion coefficients. For example, focal planes used for the detection of radiation require arrays of photodiodes which are constructed of special semiconducting alloy compounds such as PbSnTe. The output from these photodiodes must then be coupled to signal receiving and processing circuits which utilize highly developed silicon integrated circuit technology. However, there is a large thermal mismatch (approximately 18.times.10.sup.-6 /.degree.K) between silicon and PbSnTe. If a large array of PbSnTe photodiodes are directly bonded to a silicon chip utilizing prior flip-chip bonding, the thermal stresses created by temperature changes either during the processing of the chip or during its application will cause failure of many of the bonds.
According to the prior art, linear arrays of photodiodes have been supported upon circuit boards and connected by long leads to processing circuits. Such prior art indirect mating is suitable only for linear arrays and cannot provide a high density planar array of sensors directly coupled to a receiver, such as provided by the present invention.